The present application relates to telecommunication systems, and more specifically, to a method and apparatus for filtering and controlling the peak-to-average ratio of a transmitted signal. A precoder comprising a modulo operator, a feedback filter, and a peak-to-average ratio (PAR) controller provide a novel means for processing a data symbol in a data communication transmitter. The precoder has an output with a limited magnitude and a predetermined spectral shape. Several benefits are simultaneously provided by the precoder including a controlled PAR, an attenuation of unwanted frequencies and a reduction in processing complexity.
It is well known by those skilled in the art that the performance of a data communication system may be significantly improved by using Tomlinson precoding (xe2x80x9cprecodingxe2x80x9d). Typically, preceding eliminates much of the distortion, or intersymbol interference (ISI), caused by the transmission channel coupling a transmitter and a receiver. U.S. Pat. No. 5,162,812, for example, discloses a generalized partial response precoder (GPRP) device in the transmitter that performs generalized partial response signaling (GPRS) precoding for preventing certain intersymbol interference from being injected into a received signal.
Although there are alternatives to preceding, such as decision feedback equalizers (DFE""s), a precoder has advantages in certain data communication systems. A primary advantage of preceding, for example, is the elimination of error propagation. Precoding can therefore be used with coding schemes, such as trellis coded modulation (TCM), to achieve realizable and reliable soft-decision decoders.
A precoder is usually initialized during a training mode by sending a training signal from a transmitter to a receiver having a DFE. During the training mode, DFE coefficients are generated in the receiver and then transferred back to the transmitter via a return or back channel coupled to the transmitter. The transferred DFE coefficients are then used by a precoder feedback filter and are typically referred to as precoder coefficients or xe2x80x9cprecoder tapsxe2x80x9d.
In a conventional data communication system, a precoder receives a data coded signal and converts it into a precoded signal. The data coded signal may be generated using a variety of modulation schemes, such as pulse amplitude modulation (PAM), quadrature amplitude modulation (QAM), carrierless AM/PM modulation (CAP), discrete multi-tone (DMT) or other schemes known to those skilled in the art. The precoded signal then passes through a digital-to-analog converter, a shaping filter and other transmitter circuits.
A principal disadvantage of a conventional precoder is that no means are provided for controlling the peak-to-average ratio (PAR) at the output of the transmitter line driver. If the transmitter line driver output has a high PAR, then the transmit circuits of the transmitter may clip some of the peaks from the precoded signal. If frequent clipping occurs, the bit error ratio (BER) associated with the received signal at the receiver may increase and thus degrade the performance of the communication system. Therefore, it is desirable to have a communication system capable of limiting or controlling the peak value of the transmitter line driver output.
It is also desirable in some transceivers having echo cancellation for the output thereof to have both a low PAR and low values for low frequency components (including DC). For example, a high-pass filter having both a DC null of around xe2x88x9215 decibels and an output with a limited value may meet the specifications for many transmitters. Typically, a shaping filter cannot provide a low value for PAR, but can provide low values for low frequencies and DC. Hence, there is a need to have an apparatus for providing a low PAR while simultaneously keeping the DC null at or below a specified value.
Further, it would be desirable for such an apparatus to be easily implemented within the structure of an existing transmitter.
Therefore, an object of the present invention is to use a precoder to limit the maximum output of the transmitter line driver. When the peak output of the transmitter line driver is reduced, then the specifications of the line driver may be relaxed thereby providing a reduction in cost for the transmitter circuits.
Another object of the present invention is to provide spectral shaping within the precoder, thereby reducing the need for a separate spectral shaping filter. When spectral shaping is provided within the precoder, the processing requirements of a digital signal processor in the transmitter may be reduced.
Yet another object of the present invention is to provide spectral shaping within the precoder without reducing the performance of the data communication system.
It is a further object of the present invention to take advantage of the properties of modulo devices within the transmitter and receiver. For example, using modulo devices in the transmitter and receiver in conjunction with the present invention provides good data communication performance.
The above objects indicate a need for an improved method and apparatus for performing precoding functions. Further, the apparatus and method should be cost effective and have parameters that may be changed to meet various user requirements.
Hence, a precoder is provided that includes: a summer for combining a data symbol and a filtered precoded symbol and outputting an adjusted data symbol; a modulo device having an upper boundary and a lower boundary for receiving the adjusted data symbol and outputting a bounded modulo signal; and a peak-to-average ratio (PAR) controller coupled to the modulo device for generating a control signal, combining the control signal with the modulo signal so as to selectively adjust the bounds of the modulo signal, and outputting the precoded signal, wherein the combined modulo signal and control signal is the precoded signal. A precoder filter coupled to the PAR controller in a feedback arrangement is further provided for outputting the filtered precoded symbol to the summer. Advantageously, the PAR controller can be arranged so as to provide high-pass filter, low-pass filter or band-pass filter functionality.
In another aspect of the present invention, a method of precoding a data symbol is provided having the steps of: receiving the data symbol; combining the data symbol with a filtered precoded signal to generate an adjusted data symbol; performing a modulo operation on the adjusted data symbol to provide a modulo signal; modifying the modulo signal according to an algorithm wherein the output of the modifying step is a precoded signal representing the data symbol; and filtering the precoded signal to generate a new value for the filtered precoded signal.
In addition, a method of simultaneously controlling the peak-to-average ratio and the spectral shape of a precoded signal to be transmitted over a communication channel is provided. The method includes the steps of: receiving a data symbol; combining the data symbol with a filtered precoded signal to generate an adjusted data symbol; performing a modulo operation on the adjusted data symbol to provide a modulo signal; selectively adjusting the value of the modulo signal in accordance with a PAR control algorithm, thereby generating a new value for the precoded signal wherein the new value has a predetermined maximum magnitude; filtering the precoded signal to generate a new value for the filtered precoded signal; and coupling the precoded signal to transmitter circuits for transmission over the communication channel.
Further objects, features and advantages of the invention will become apparent from the following detailed description taken in conjunction with the accompanying figures showing illustrative embodiments of the invention.